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Dave Cook is a voice-over artist who creates digital sound files at home and distributes them over the Internet.

Yesterday, he had to turn to the computers at the Concord public library to get the job done. Without Internet service, he said, “I’m kind of dead in the water.’’

Cook was one of thousands of Comcast Corp. customers in New England and Washington, D.C., who were cut off from the Internet for at least three hours, beginning at 8 p.m. Sunday. For some, the unusual outage extended deeply into Cyber Monday, one of the heaviest days of the year for online shopping. Cook’s service, for example, was not restored until after 3 p.m. yesterday.

Customers who called Comcast Sunday night to ask about the outage were met with busy signals, dropped calls, and automated voice messages urging them to consult the company’s website, even though they were unable to connect to the Internet. And yesterday Comcast declined to furnish answers to basic questions: How many customers lost service? What precisely caused service to get cut off?

Comcast spokeswoman Doreen I. Vigue said only that the company has more than 2.5 million subscribers to its video services in New England, and that its telephone and cable television service was not affected.

Messages acknowledging the outage were available on the company’s phone-in customer service lines “within minutes of discovering there was a problem,’’ she said, adding they did not direct customers to the company website. “That should not have been the messaging people heard. If they did, it was a mistake, and it was corrected early on,’’ Vigue said.

As for the outage, “Comcast apologizes for the inconvenience to our customers,’’ she said.

The disruption of Comcast’s Internet service appeared to originate in its DNS, or Domain Name Server system, which translates website names into digits that computers can process. Such outages seem to be rare for the Comcast network. Preston Gralla, contributing editor at Computerworld magazine and author of the book “How the Internet Works,’’ remembered just one such incident in more than 10 years as a Comcast customer.

DNS-related failures are often caused by hackers trying to disrupt networks, Gralla said. Comcast declined to comment on that possibility.

The service interruption presented a customer service challenge for the cable company. A breakdown in Internet service makes it difficult for customers to find the support they need, which is usually available online.

Numerous users switched to their smartphones to access the Internet, and shared information on social networks about how to route around the problem.

Jared M. Spool, founder of consultant User Interface Engineering in North Andover, was working from home when his Internet connection disappeared. “It went out and I said, ‘OK, I guess I’m not supposed to work,’ and I went and cooked dinner,’’ Spool said. “And then after dinner, it was still out, so the first thing I did was try to call Comcast. And it was just pure busy.’’

Eventually Spool pulled out his Verizon Wireless 3G Wi-Fi box, which uses the cellular phone network, and used that to log on to the Internet. He also sent a complaint out on Twitter: “Comcast Boston area is down. On the 3G for now. It’s like a candle in a blackout.’’

Over Twitter, he received a suggested fix: a procedure to connect his computer to a Google DNS server, and bypass the Comcast problem. “Sure enough, it worked immediately,’’ Spool said.

Other users just waited it out.

Thomas A. Stone of Falmouth, a senior research associate with The Woods Hole Research Center, was checking his e-mail from home, hoping to get a jump on the workweek, when he lost his Comcast Internet service.

“I assumed some virus had jumped onto my machine,’’ he said. Later, when Stone tried calling Comcast, he realized the problem might be more widespread. He couldn’t get through to the help line.

Stephanie Guarino and her husband, Justin Wanamaker, were at home in Attleboro Sunday night, checking their fantasy football standings and trying to connect to a YouTube channel. When they lost Internet service, they tried to find a number to call Comcast — not so easy without the Internet. “It was brutal,’’ Guarino said, adding that once they got the number, they couldn’t get through to Comcast. After repeated busy signals, they finally connected — and were cut off.

Jeffrey Schiller, a longtime network manager at the Massachusetts Institute of Technology, quickly identified the Comcast outage as a DNS problem after his wife told him they had lost Internet service. Schiller is also a member of the Internet Engineering Task Force, an international organization that manages Internet standards and protocols. Schiller said DNS-related outages are “not usual, but are usually resolved more quickly.’’